Line edge roughness (LER) or line width roughness (LWR) is a fundamental challenge in the semiconductor industry.
LWR on transistor gate length is a dominant parameter for determining the variability of threshold voltage, on-current
and off-current, and LER on interconnect impacts breakdown voltages. Integrated circuit (IC) scaling enabled by
lithography is the technology to increase device density and improve performance. However, scaling below 32nn
technology node induces short-channel effect (SCE) because of the short distance between the transistor source and the
drain. Final LER on the working layer results from several processes, and, thus, LER controls rely on lithography, resist
properties, post-resist-development processing, pattern transfer methods, and photomask LER. Now, pattern generation
is main source of LER, where short exposure wavelength and consecutive low photon numbers result in discrete photon
flux and shot noise, causing high LER. Extreme ultraviolet lithography (EUVL) uses shorter wavelengths and a lower
dose than the current 193 nm lithography, making LWR one of the three most critical challenges. Characterization of
LER is also a challenge. The current rms method is broadly used; however, this approach is not enough and a better
method has yet to be established.
An overview of extreme ultraviolet lithography (EUVL) mask etch is presented and a EUVL mask etch study was carried out. Today, EUVL implementation has three critical challenges that hinder its adoption: extreme ultraviolet (EUV) source power, resist resolution-line width roughness-sensitivity, and a qualified EUVL mask. The EUVL mask defect challenges result from defects generated during blank preparation, absorber and multilayer deposition processes, as well as patterning, etching and wet clean processes. Stringent control on several performance criteria including critical dimension (CD) uniformity, etch bias, micro-loading, profile control, defect control, and high etch selectivity requirement to capping layer is required during the resist pattern duplication on the underlying absorber layer. EUVL mask absorbers comprise of mainly tantalum-based materials rather than chrome- or MoSi-based materials used in standard optical masks. Compared to the conventional chrome-based absorbers and phase shift materials, tantalum-based absorbers need high ion energy to obtain moderate etch rates. However, high ion energy may lower resist selectivity, and could introduce defects. Current EUVL mask consists of an anti-reflective layer on top of the bulk absorber. Recent studies indicate that a native oxide layer would suffice as an anti-reflective coating layer during the electron beam inspection. The absorber thickness and the material properties are optimized based on optical density targets for the mask as well as electromagnetic field effects and optics requirements of the patterning tools. EUVL mask etch processes are modified according to the structure of the absorber, its material, and thickness. However, etch product volatility is the fundamental requirement. Overlapping lithographic exposure near chip border may require etching through the multilayer, resulting in challenges in profile control and etch selectivity. Optical proximity correction is applied to further enhance the resolution. Other resolution enhancement techniques, such as phase shifting, are also in consideration for EUVL. Phase-shifting will involve partial etching of the multilayer. The trend to use shorter EUV wavelength (e.g., 6.7 nm) for enhancing resolution will use new multilayer and absorber compositions, and will require new etch process development efforts. TaBO/TaBN absorber layers (features down to 40 nm) were etched with vertical profiles, low etch CD bias, and 1.7 nm etch CD uniformity (3σ ). In the light shed application, Mo/Si multilayer etching yielded vertical profiles and high etch selectivity.
Mask defectivity is often highlighted as one of the barriers to a manufacturable EUV solution. As EUV lithography
matures, other components of mask making also emerge as key focus areas in the industry: critical dimension (CD)
control, film variability, selectivity, and profile tolerance. Mask materials and specifications continue to evolve to meet
the unique challenges of EUV lithography, creating the need for etch capabilities that can keep pace with the latest
developments. In this study, the performance of a new EUV mask etch system will be evaluated using a variety of mask
blanks to determine the relative performance of each blank type. Etch contributions to mean to target (MTT), CDU,
linearity, selectivity, capping layer uniformity, line edge roughness (LER), and profile quality will be characterized to
determine tool performance. The new system will also be used to demonstrate multilayer etching capabilities, important
for opaque frame and alternating phase shift applications. A comprehensive summary of the etch performance of various
EUV films and the readiness for manufacturing applications will be provided.
High particle removal efficiency (PRE) up to 99%+ without damage to sub-50 nm linewidth features has been
demonstrated using a mixed fluid jet technology and sulfur-free chemistry. This high PRE was achieved with several
types of deposited particles, including polystyrene latex spheres. Damage-free cleaning was demonstrated on binary and
phase shift masks with Cr and MoSi structures. All masks were processed using the TetraTM mask cleaning tool
configured with the NanoDropletTM mixed fluid jet technology.
Studies on the haze formation mechanism in deep ultraviolet (UV) lithography were carried out. A nucleation and
growth model on haze formation was proposed based on Gibbs energy change analysis. Haze formation is a nucleation
and growth process. For a spontaneous process, Gibbs energy change in nucleation results from surface energy increase
and volume energy decrease; the former tends to prevent nucleation and the latter to enhance it. Then, a critical
dimension of nuclei exists, i.e. when a nucleus is larger than critical dimension, it can grow with a system Gibbs energy
decrease. The study shows that it is thermodynamically spontaneous for small haze to merge with large haze. A critical
distance exists, beyond which nuclei grow independently. The haze formation region on a plot of composition versus
temperature was proposed based on the equilibrium constant calculation and a haze growth mechanism postulated.
Photochemical dissociation of molecular oxygen to atomic oxygen by excimer laser less than 246 nm wavelength plays
an important role in the oxidation of sulfur dioxide. The proposed mechanism agreed well with previously published
Etching of TaBO/TaBN absorbers on EUVL masks was studied. The self-mask strategy and etch selectivity optimization
were used for obtaining the best etch CD performance. Gibbs Energy Minimization was used for determining etch gas
selection and product volatility. Calculated results suggest the use of a two-step etch process, i.e. using fluorinecontaining
gas to etch the antireflective (AR) layer and using chlorine-containing gas to etch the bulk absorber beneath
AR. High selectivity of TaBN-to-TaBO was obtained and the AR hard mask function was proven. By using this method,
one EUVL mask can be used many times by selectively exposing portions of a mask during etch. A profilometer was
used for etch product characterization and etch CD results were verified by using CD SEM measurement. Optimal
conditions developed on the Applied Materials Tetra Mask Etch System by using just one mask gave etch CD bias of 3
nm, etch CD uniformity of <3 nm, excellent sidewall profile, and high selectivity of absorber layer to resist and absorber
to buffer layer. Etch effects on the backside chrome coating were also examined. No arcing on the backside during
EUVL absorber and buffer etching was identified.
Studies on pattern transfer of next generation lithographic (EUV) photomask were carried out. Based on current absorber layer material candidates, thermodynamic calculations were performed and plasma etch gas system and composition were investigated. The gas systems have the advantage of all etch products being in volatile condition. This is helpful to keep the etch process and etch chamber clean. For etch CD bias challenge in EUV photomask etch, self-mask concept was investigated, which makes anti-reflective (AR) sub-layer of the absorber layer function as a hard mask for the bulk absorber layer beneath. It significantly reduces etch CD bias and improves pattern transfer fidelity. For common candidates of EUV mask absorber layers such as TaBO/TaBN and TaSiON/TaSi, reactive gas systems were proposed according to thermodynamic calculations with all products volatile. AR sub-layers were etched in one gas composition with volatiles. Once the AR sub-layer is etched through, gas composition was changed so that the bulk absorber sub-layer beneath is etched selectively with volatile products. Excellent results in profiles, CD bias, CD uniformity, and underneath buffer/capping layer impact have been demonstrated.
A robust photomask etching process was studied and developed for 65 nm node photomask production with zero CD process bias. The fabrication process, including pattern generation and transfer do not use data sizing, saving photomask delivery time, improving yield, and reducing fabrication costs. The photomask patterns, without using data sizing cover chrome loads from about 1 percent to 80 percent. For 65 nm critical layer EAPSM, the CD bias of Cr and MoSi etching together is equal to or less than about 20 nm for high and low load photomasks. The etch process and dose adjustment on the 50 keV e-beam writer allow for zero CD process bias, i.e. the data sizing becomes unnecessary in the 65 nm node photomask fabrication. The SMIF pot utilization in both pattern generation and transfer processes significantly improved the defectivity control. Cr and MoSi etch endpoints of 1% load photomasks were clearly detected. Point-to-point CD etch contributions for dark and clear features are 5 nm (3 sigma) or less and final CD value ranges are 8 nm or less. CD etch linearity and other etch properties on SRAF and serif are also discussed. An equation was proposed for calculating phase angle non-uniformity distribution, and phase angle range can be controlled in the range of 1.4±0.3 degree. "Self-mask", i.e. using AR sub-layer as hard mask for beneath chromium sub-layer etch was also discussed.
Photomask plasma etch reactions were studied using Gibbs energy minimization method. The equilibrium compositions were analyzed at practical photomask plasma etch conditions of temperatures, pressures, and reactant inputs. The thermodynamic calculations were based on common gaseous systems used in photomask plasma etching such as Cl2-O2-He, SF6-O2-He, and CF4-O2-He, as well as alternative gases. For Cr etch, the thermodynamically calculated results showed that volatile CrO2Cl2 was the moderate equilibrium composition in the predetermined system only when the temperature was higher than 400°C, indicating that temperatures of heavy particles in practical plasma conditions might be higher than this temperature. The effects of assistant chemicals on equilibrium compositions were investigated. For MoSi etch, the thermodynamic calculation showed that the main volatile etch products were MoF6 and SiF4. The comparison of MoSi etch using SF6 and CF4 was made and gaseous input condition for obtaining all volatile products was found, which would be helpful for defectivity and passivation controls. The calculation also showed that the addition of oxygen in SF6 and CF4 systems could increase the equilibrium composition of atomic fluorine, resulting in the etch rate increase. This result agreed with previous hypothesis on the oxygen effects on etch rate. For quartz etch, the calculation showed that the main volatile etch product was SiF4. For Ta or TaN absorber EUV mask etch, the volatile Ta-containing product was found to be TaCl5.
Studies on photomask quartz etching in a gas system consisting of SF6, O2, and He were carried out using an inductively coupled plasma (ICP) photomask etcher. A profilometer was used for etch rate uniformity characterization. Quartz etch was performed in the same chamber as was used for MoSi etching. The defectivity influence was also investigated so that the feasibility of MoSi and quartz etches in the same chamber was examined and discussed. Process optimization was studied by changing the Cr layer thickness, the O2, SF6, and He flow rates and gas composition, the ICP power and bias power, and the pressure. It was found that ICP power, bias power, pressure, O2 flow rate, and He flow rate played an important role in determining etch performance. Plasma stability and etch performance were also investigated. An empirical kinetic equation was proposed based on experimental data and a quartz etch mechanism was also explained. Systematic etch non-uniformity resulting from radial and the side-to-side errors were also investigated in this study. High CD resolution (down to 100 nm), good etch rate linearity, extremely low etch rate dependency on photomask patterns with different loading, and repeatable etch rate produced an optimal etch process suitable for 90 nm and below technology node.
Studies on Cr etch and its kinetics were carried out using a 50KeV photomask e-beam writing system, an ICP plasma etcher, chemically amplified resist (CAR), and a scanning electron microscope (SEM) metrology tool. A Cr etch rate equation was developed, showing good agreement with experimental data. Both the theoretical rate equation and experimental results showed that the main Cr etch rate effect parameters were oxygen mass flow rate, oxygen partial pressure, and ICP power. It was found that pressure plays a very important role in critical dimension (CD) uniformity etch contribution, loading effects, isolated/dense (I/D) etch CD bias, and etch CD movement. Etch kinetic information was found to be very helpful for improving CD uniformity, reducing pattern (local loading) effects, and controlling CD movement at the etch step. Some obsolete-pattern photomasks were used in the kinetic study. The main advantages of using obsolete photomasks include reducing resist effects on Cr etch rate investigation, obtaining much more etch kinetics data, and significantly lowering process development costs normally incurred from lithography tool time and raw photomask material consumption.
Theoretical and experimental Cr photomask etch studies are carried out using different resists [ZEP, chemically amplified resists (CAR), and optical resists] and different brand etch tools. The effects of chrome loading are analyzed, and theoretical equations are developed for etch time calculations and endpoint determinations of extremely low Cr load photomasks. It was found that these equations agreed well with experimental data. Etch critical dimension (CD) movement data are analyzed and calculated, showing agreement with experimental data. Metrology measurement and characterization tools include a profilometer, an optical film measurement system, and SEM and optical CD measurement systems. Significant etch performance differences are noted across etch tools, irrespective of the resist type used. An etch property number method is proposed, which is found to accurately describe the etch process analysis and the extent to which etch performance can be expected to be improved. Etch properties are focused on etch CD movement, isolated/dense etch CD bias, radial CD etch contribution, and Cr load effects.
The Advanced Metrology Advisory Group (AMAG) is a council composed of the chief CD-metrologists from the International SEMATECH consortium's Member Companies and from the National Institute of Standards (NIST). The AMAG wrote, in 2002, with CD-SEM supplier involvement, updated the 'Unified Advanced CD-SEM Specification for Sub-130nm Technology (version 2002)' to be a living document which outlines the required performance of advanced CD-SEMs for supplier compliance to the 2001 International Technology Roadmap for Semiconductors, and also conveys other member companies' collective needs to vendors. Using this specification, a benchmarking effort of the currently available advanced CD-SEMs will be performed. As these results are not yet complete, they will be presented at a future date. However, the current version of the specification has undergone many changes and improvements from the last, and these will be discussed here.
Studies on MoSi etch processes were carried out using a design-of-experiments (DOE) methodology. Data was gathered using profilometers and optical critical dimension (CD) measurement systems. MoSi etch chemical kinetics was studied and a kinetic equation was developed showing that MoSi etch rate has a linear relationship with inductively coupled plasma (ICP) power, and an exponential relationship with the negative inverse of the DC bias electrode voltage. The reaction orders of the MoSi etch were 0.2 to SF6 concentration (molar fraction) and 0.5 to O2 concentration, and the activation energy was found to be 1350 kJ/mol. A dimensionless number method was used for results analysis and a calculable dimensionless number was defined. This number was found to be proportional to the isolated/dense etch bias. Etcher performance was also analyzed using a matrix transformation method and it was found that matrix-analysis-calculated etch results agreed with those obtained by experimental measurement. The optimal MoSi etch process window was expressed graphically.
Mathematical prediction of dry etch results have been difficult, if not impossible, over the years due to the complexity inherent in plasma processing. With device requirements tightening combined with the high cost of running test masks, it becomes even more important for process engineers to have some means by which to model accurately this increasingly complex activity. In this paper, a correlation between a novel, plasma chrome etch model and empirical data will be presented. Practical effects of chrome loading are also discussed.
From the empirical data side, studies on the photomask chrome etch process using e-beam resists were carried out following strict design of experiments (DOE) format. Photomask chrome etch processes with different resists (ZEP and positive CAR) were investigated using a Unaxis VLR 770 ICP etcher and an AMAT Tetra etcher. A Toshiba EBM-3500B (50KeV) was used for the pattern writing on positive CAR resist. A KLA-Tencor P12-EX profiler, Leica LWM 250 UV CD optical measurement system, and a KLA-Tencor 8250XP-R CD SEM were used for metrology and product characterization. Significant etch performance differences were shown on these etchers irrespective of the resist type used. A special measurable number was defined and found to have a linear relationship with etch movement on both AMAT and VLR etchers.